1. Field of the Invention
The present invention generally relates to highly conductive thermoplastic composites intended for the rapid and economical production of fuel cell bipolar plates, and the method for making the same.
2. Background Description
Bipolar plate materials have historically been metals coated with corrosion resistant layers or graphite with a seal treatment to lower the gas permeability. In both cases, the bipolar plates require extensive machining and post processing, resulting in hardware costs far more expensive than the costs for the raw materials alone. To date, the costs of bipolar plates dominate the stack costs. Unless bipolar plates that are considerably less expensive are developed, PEM (Proton Exchange Membrane or Polymer Electrolyte Membrane) fuel cells cannot easily be applied to civilian markets to compete with established power technology.
As one of the key and costly components of PEM fuel cells, the bipolar plates must have high electrical conductivity, sufficient mechanical integrity, corrosion resistance, low gas permeability, and low-cost in both materials and processing when applied to the civil market. To replace graphite bipolar plates and lower the cost, a variety of composite bipolar plates have been developed. Most of them were made by compression molding of polymer matrices (thermoplastic or thermoset resins) filled with conductive particles (such as graphite powders) or fibers. Because most polymers have extremely low electronic conductivity, excessive conductive fillers have to be incorporated, and it is very difficult to get high conductivity and sufficient mechanical properties at the same time. To solve this problem, Oak Ridge National Laboratory (ORNL) recently developed carbon/carbon composite bipolar plates. The manufacture process consists of multiple steps, including the production of carbon fiber/phenolic resin preforms (by slurry-molding or wet-lay process) followed by compression molding, and the pyrolysis and densification on the surface by a chemical vapor infiltration (CVI) process. The plates have high conductivity (about 200-300 S/cm), and the technology has been transferred to Porvair Fuel Cell Technology for scale up. However, this process is too complicated and is by no means economic.